1. Field of the Invention
This invention relates to silicon production and more particularly to processes for producing high purity silicon. Specifically, this invention relates to a process for refining impure silicon, such as metallurgical grade silicon, to form sheets of high purity silicon usable in photovoltaic solar cells and the like.
2. Description of the Prior Art
High purity silicon is both useful and oftentimes required for a wide variety of industrial applications. One such application is in the area of photovoltaic solar cells. In such photovoltaic cells, thin sheets or wafers of highly refined silicon form at least the upper surface of a multi-layered cell adapted for direct conversion of incident solar radiation to an electrical potential. To date, the processes and techniques available to produce high purity silicon and to subsequently form such silicon into thin sheets or wafers are exxtremely expensive.
One commonly used technique for electrochemically purifying silicon is an adaptation of a well-known aluminum refining process. In this technique, silicon is substituted for aluminum to provide an electrochemical process utilizing a molten Cu/Si anode with Na based molten electrolyte. The electrolyte contains Na.sub.3 AlF.sub.6 for transporting silicon to a cathode. An example of such a process is disclosed in the French paper by R. Monnier and J. C. Giacometti entitled, "Recherches sur la Raffinage Electrolytique du Silicium", Helvetica Chimica Acta, Vol. 47,345, (1964).
There are several significant problems with the above described type of system. One such problem is that since the anode must be molten, only a single electrode pair per electrochemical cell can be utilized. Therefore, silicon deposition per unit cell volume on the cathode proceeds rather slowly. Furthermore, due to the dynamics of such cells, large electrolyte vapor losses result from the high cell operating temperatures. Consequently, this particular system is not very economical.
Another known electrochemical process for plating Si is described in a paper by Uri Cohen entitled, "Some Prospective Applications of Silicon Electrodeposition from Molten Fluorides to Solar Cell Fabrication", J. Electronic Mat'ls, Vol. 6, #6, 607 (1977). In this particular process, a LiF, KF, K.sub.2 SiF.sub.6 molten salt electrolyte is used to plate silicon onto a graphite cathode. However, a substantially pure solid sheet Si anode is required. Thus, this technique has no practical application for use with metallurgical grade or other impure silicon.
Other techniques presently being utilized to produce highly pure silicon for the semiconductor and photovoltaic industries include well known distillation processes as well as various chemical conversion techniques. However, all of these techniques are rather expensive to operate in order to produce sufficient quantities of highly pure or refined silicon. Therefore, there is a definite need for an economical process for producing highly refined silicon from metallurgical grade or other impure silicon.
Once highly refined silicon is produced, it must then be formed into thin sheets or wafers to be useful in the photovoltaic industry. The presently available processes for wafer or sheet formation are themselves very expensive to operate. Two common techniques for forming sheets of silicon are rolling and casting. One disadvantage with most rolling techniques is that silicon can only be rolled near its 1400.degree. C. melting point, and this temperature is generally too high for fine rolling at production rates. Furthermore, the resultant silicon tends to be very brittle. A major difficulty with casting techniques, which are more frequently used, is that molten silicon reacts with most materials to some degree. Therefore, molds containing molten silicon are very expensive, and some of the silicon is lost due to removal of those portions of the casted silicon containing impurities from the mold casing.
A third commonly used technique for forming sheets of silicon comprises the cutting of sheets or wafers from solid blocks of silicon. However, such Si sheets must be very thin, and this feature results in a great deal of waste (up to 50%) in the form of silicon sawdust. This waste of highly pure silicon is, obviously, very uneconomical.
One of the major problems and hurdles facing the photovoltaic solar cell industry is in reducing the costs associated with producing photovoltaic cells. Efficient photovoltaic cells utilizing silicon are in fact presently available and could readily be utilized on a mass basis if it were not for a prohibitive pricing structure. Much of this pricing structure is a direct result of the high cost of purifying silicon and forming such purified silicon into thin wafers or sheets. The present invention meets both these needs of the photovoltaic industry by providing highly refined silicon at an economical cost as well a providing such highly refined silicon in sheet form at no substantial additional cost.